This invention relates to image-shearing apparatus, as used, for example, in sizing instruments.
One of the most important applications of image-shearing apparatus lies in the field of microcircuit manufacture, where image-shearing is used for the precise and accurate measurement of linewidths on semiconductor wafers and the masks used in their production. Image-shearing microscopes are described in United Kingdom Patent Specifications Nos. 910,319; 948,273; 1,285,184 and 1,375,036. An improved split-field image-shearing microscope is described in recent U.S. patent application Ser. No. 060,868. The present invention utilizes the same underlying principle as exploited in the latter application, namely the eye's ability to detect a very small abrupt, step-wise displacement of one segment of a line relative to another immediately abutting segment.